1. Field of the Invention
The present invention generally relates to communication devices and more specifically to optical transmission devices for providing two-way communication.
2. Description of the Related Art
Japanese Unexamined Patent Laid-Open No. 5-133716 discloses a conventional optical transmission device configured to perform two-way communication by using two communication devices that spatially separated from each other. FIG. 5 shows such an optical communication device A that communicates light beams LA as well as receives light beams LB from another communication device B (not shown).
In operation, a laser beam is emitted from a laser diode 101 and propagated as linearly polarized light through a lens group 102. Thereafter, it is reflected from a polarizing beam splitter 103, and then reflected by a variable-angle mirror 104a of an optical axis adjusting unit 104 to the device B.
Similarly, the received light beam LB from device B is reflected by the variable-angle mirror 104a, through the beam splitter 103 to branching element 105. A substantial portion of the light beam LB is transmitted through the branching element 105 to a photodetector 106 by a lens group 107. The other portion of light beam LB is reflected from the branching element 105 to a photodetector 108, which is a position photodetector, via a lens group 109. A substantial portion (LBa) of the received light beam LB is transmitted through the beam branching element 105, and is converged onto a photodetector 106 by a lens group 107. The photodetector 106 acts as a real signal photodetector. The other portion of light beam LBb reflected from the beam branching element 105 is converged by a lens group 109 as a luminous flux which is received by a photodetector 108.
In order to achieve the most efficient transmission and reception of light, an optical axis 112 on the beam splitter side, which corresponds to the common optical axis for transmission and reception, can be backwardly inclined so that the directions of the transmitting light beam LA and the received light beam LB form right angles with respect to each other.
For high-capacity communication, a small element having an effective light receiving area of less than 1 mm, such as an avalanche photodetector, must be used as the photodetector 106. And, the positions of the photodetector 106 and the position detecting photodetector 108 are aligned so that the light beam LB falls on the effective receiving area of the photodetector 106. The variable-angle mirror 104a is adjusted so that the optical axis of the light beam LB is at the center of the photodetector 108.
For efficient communication, the optical axis of the light beam LA is aligned with the center of the photodetector 108. A spot SP generated on the surface of photodetector 108 by light beam LB, provides a misalignment infonnation signal that is received and processed by a signal processing unit 110, which is then transmitted to a mirror drive control unit 111 to generate a correction signal. Based on this signal, the angle of the variable-angle mirror 104a is adjusted to continuously align the optical axes of the light beams LA and LB.
The photodetector 108 generally employs a quadrant photodetector, which is divided into four elements 121 by a separation area 122 as shown in FIG. 6. The method for detecting a position using a photodetector has been described in e.g., Japanese Laid-open patent 2001-94513. Such a photodetector 108 is arranged so that the light receiving surface (plate) of the quadrant photodetector is generally located in a position defocused to a converging point of the lens group 109.
However, the optical transmission device, which transmits and receives light beams through the atmospheric air in the related art described above is affected by a phenomenon in which the transmitted light beam fluctuates due to microscopic fluctuations in the air.
FIG. 7 is an explanatory drawing showing modeled microscopic fluctuations, in which the distribution of strength of the transmitting light fluctuates in the atmosphere. The symbol W designates the width of light beam LA from device B. Since atmospheric air is inhomogeneous, the refractive index varies spatially and temporally. When an air layer partially having a high refractive index exists in an optical pass of the transmission light LA, the portion of the high refractive index works as a convex lens, and thereby generates a light-concentrating effect and point W1, which is high in intensity, and point W2, which is low in intensity, are generated in the width W of the transmitting light beam LA at the position of the receiving device A.
Also, since the distribution of intensity varies temporally, point W2 appears to fluctuate within width W, a phenomena known as microscopic fluctuation. A disadvantage of the related art is that since the light receiving surface of the photodetector 108 is set at a position defocused from the converging point during microscopic fluctuations of the atmospheric air, the distribution of light intensity in spot SP becomes uneven.
In FIG. 7, the distribution of light intensity at the beam entrance M of the device (the entrance pupil), is projected as shown. Consequently, the spot SP having an adequate area on the light receiving surface is as shown in FIG. 8.
As shown in FIG. 9, the spot SP having a diameter T, hatched portions P1 of high-intensity and portions P2 of low intensity are generated, and the center of light intensity PC, which differs from the center of luminous flux BC, is determined to be the optical axis. Therefore, misalignment of the direction of the optical axis of the transmitting light beam LA occurs by an angle corresponding to an amount of misalignment S, and consequently, the transmitting light beam LA is deviated from the device B, which can cause interruptions in the communication system.
To solve the above-mentioned problems, it is preferable that photodetector 108 is arranged in a position adjacent to the converging point of the lens group 109 and the size of the spot SP is arranged so as to be less than the minimum resolution of the device. However, the light beam can intersect separation area 122 between each of the divided elements, and when the spot SP crosses over the light beam intersects separation area 122, the output from the photodetector 108 suddenly becomes low and is stopped in the worse case.
In such a case, although the optical axis actually exists on the photodetector 108 and the communication is normally and rightly being conducted, the system wrongly detects that the optical axis has been misaligned and moves the mirror 104a so as to align the optical axis. Thereby the optical axis existing on the photodetector 108 is shifted out of the correct range and the communication is terminated.